The elder Roger then joined the rebels at the start of the First Barons' War in 1215, being captured in November at the siege of Rochester Castle, paying 250 marks for his release.

After the death of the elder Roger some time before 1251 his son William de Leybourne inherited seven Knight's fees in Kent and Oxfordshire, as well as substantial debts, which were only cancelled in 1253 by Henry III.

By August 1263 the Marcher Lords were in negotiations with Edward, having been unnerved by de Montford's plan to ally with the Welsh, and they swore an oath to the king on the 18th.

During the second half of the conflict with de Montfort, Leybourne fought at the Battle of Northampton and as a defender at the siege of Rochester Castle, where he was badly wounded.

In November he fought the rebels in Kent, and in January 1266 he recaptured Sandwich, serving as Edward's deputy for the capture of the other Cinque Ports.

He went on a second pilgrimage in 1269, and was rewarded with 1000 marks from Ottobuono, the papal legate, who later became Pope Adrian V. Rather than going to the Holy Land he travelled to Gascony, where he had been appointed Lieutenant on 29 November 1269, with his aim possibly being to raise men for the crusade.